Critical undercooling for different primary phase formations in Nd15Fe77.5B7.5 alloy

Summary form only given Primary /spl gamma/-Fe dendrites are often present in as-cast NdFeB alloys, and can deteriorate the magnetic properties of sintered magnets significantly. In order to solve this problem, the present authors have applied the drop tube processing technique to a number of peritectic NdFeB alloys. It was found that primary /spl gamma/-Fe formation is suppressed in most of droplets owing to significant undercoolings achieved prior to solidification. As a result, primary solidification either of the peritectic Nd/sub 2/Fe/sub 14/B phase or of the metastable Nd/sub 2/Fe/sub 17/B. (x=0-1) phase has been observed, depending on the alloy composition and on the droplet size. In order to determine the critical undercooling level for different primary phase formations, the present work applied the electromagnetic levitation technique to bulk droplets of a Nd/sub 15/Fe/sub 77.5/B/sub 7.5/ alloy. By means of a pyrometer, the sample temperature was measured as a function of time, thus allowing the determination of the sample undercooling. After the levitation experiment, the sample was examined using a scanning electron microscope (SEM). Then the critical undercooling level was determined by correlating the microstructure of the sample to the undercooling level.